APHIDS ADVANCE

lynn grooms | Jul 01, 2005

THEY'RE BACK, and they're fast. By mid June, entomologists already had reported sightings of soybean aphids in the northern soybean belt. What's more, temperatures in the 70s to 80s and dry weather in much of the Midwest were providing the right conditions for further infestation. But with new techniques, such as “speed scouting,” and an arsenal of foliar insecticides, soybean growers have some tools to keep pace with a pest whose populations can more than double in just a few days and put more than a dent in yield potential.

University of Minnesota entomologists developed the speed-scouting method to get crop scouts in and out of fields quickly. “It is essentially based on a simple decision: Does the plant have more than 40 aphids on it?” explains Ken Ostlie, University of Minnesota entomologist.

If a plant has fewer than 40 aphids on it, it should be considered noninfested; if it has 40 or more, it should be considered infested. At www.soybeans.umn.edu, the University of Minnesota provides a printable worksheet that growers can use to determine whether to treat a field or to resample it in three to four days. The speed-scouting method is designed to improve the cost (in time) of sampling because every insect need not be counted.

The university is quick to point out, however, that this is not a new economic threshold. Based on several years of study, the majority of university and industry entomologists recommends a threshold of 250 aphids/plant through pod set for most Midwestern soybeans. The speed-scouting plan uses the percentage of infested plants as an indicator of damaging soybean aphid populations.

Long residual

The University of Illinois recommends that growers treat at the 250 aphids/plant threshold. Kevin Steffey, University of Illinois entomologist, says that all of the currently available insecticides are effective, but adds that pyrethroids and chlorpyrifos (Lorsban-4E, Nufos) offer the longest residual in Illinois. The pyrethroids include Asana XL, Baythroid, Mustang Max, Proaxis and Warrior with Zeon Technology.

Steffey adds, however, that in areas with a lot of dry weather, spider mites also may be present. In this case, growers may be advised to use Lorsban-4E, an organophosphate, to control both pests. Organophosphates for control of soybean aphid include Lorsban-4E, Penncap-M and Nufos (a generic liquid chlorpyrifos).

Application

DuPont recommends application of Asana XL when aphids hit the 250-per-plant threshold. Growers can apply the insecticide at 5.8 to 9.6 oz./acre, beginning applications mainly at the R1 to R2 stage. Growers are encouraged to stay aware of local conditions and recommended scouting methods because aphids may reach threshold prior to the reproductive stages, Swanson says.

Like DuPont, Cerexagri, maker of Penncap-M, recommends that growers follow university guidelines for thresholds. Penncap-M is a microencapsulated product that can get down through the canopy. It is not a fast-releasing compound; instead it doses out over a 7- to 10-day period at the recommended rate of 1½ to 2 pints/acre, says Phil Robinson, manager, field development group, Cerexagri. In addition to controlling soybean aphid, Penncap-M helps control bean leaf beetle, corn rootworm beetle and Japanese beetle. The Japanese beetle has recently become more of a problem in eastern Illinois.

Dave Ruen, product technology specialist, Dow AgroSciences, says that Lorsban-4E's fuming action allows it to get well down under the canopy and disperse along the undersides of leaves to provide contact activity against soybean aphid. When pest numbers hit the 250-per-plant threshold, Lorsban-4E may be tank mixed with glyphosate or other postemerge soybean herbicides. It also may be tank mixed with fungicides or micro nutrient sprays, Ruen says.

Dow AgroSciences, through a joint venture company, also manufactures Proaxis, a pyrethroid for soybean aphid control, which is sold through select distributors.

FMC Corporation offers two insecticides for soybean aphid control: Mustang Max, a pyrethroid; and Furadan 4F, a carbamate. “We recommend treating at 50 to 100 aphids/plant to get better coverage and to knock populations down,” says Hooten, pointing to South Dakota State University research. (For more information, see agbionews.sdstate.edu/articles/catangui112404.htm.) FMC reports that insect pressure does not always mean a particular pest must reach an economic threshold before a grower acts. Multiple pests in the aggregate can cause threshold impact. The company adds that Mustang Max has extended residual activity providing more days of control.

Hooten adds that Mustang Max and Furadan may be mixed to help prevent resistance. “If farmers are not careful with chemical compounds, they could have resistance problems in the future,” Hooten says.

Natural enemies

When outbreaks of soybean aphids occur, growers can either spray or wait it out and hope that natural enemies take action, Steffey says. In fact, there is a multistate effort to find more safe and effective natural predators of the soybean aphid. The research is a collaboration of Purdue University, the University of Wisconsin, Michigan State University, the University of Minnesota, Iowa State University, the University of Illinois, the Illinois Natural History Survey and the USDA. It is funded by the North Central Soybean Research Program.

A team of researchers, led by Purdue University entomology professor Bob O'Neil, is returning this summer to Japan, China and Korea where it already has identified seven species that kill soybean aphids. The researchers will try to better understand predators (such as parasites and fungal diseases affecting the soybean aphid) that are more specific to soybean aphid than, say, the multicolored Asian lady beetle, which has become a nuisance in residential areas. The researchers are searching for natural enemies that also will rid fields of soybean aphid without harming other ecologically important insects.

“Most of the soybean aphid's natural enemies we are studying are parasites,” O'Neil says. “They lay their eggs inside the aphids. The egg hatches into a little maggot-like larva, which eventually pupates, turning the living aphid into a mummy. A new adult parasite emerges from the pupa/mummy, and the cycle continues. If you've seen the movie Alien, you have the general idea of the parasitic lifestyle of these insects.”

The natural enemies that researchers identified on previous trips to Asia are currently quarantined at the University of Minnesota and the USDA Agricultural Research Service lab in Newark, DE. Researchers will test them further to understand their impact. “If all cards fall into place, we will do small-scale releases. But first we will need federal permits and state approvals to do them,” O'Neil says. After that, possibly within the next five years, the researchers will do more widespread testing.